The purpose of this project is to characterize an SV40 induced 53,000 MW (p53) cellular protein in embryonal carcinoma cells (EC), embryonic cells and in various transformed cells. Undifferentiated EC cells and primary cells prepared from midgestation mouse, rat and hamster embryos all expressed the protein p53 without SV40 infection. The 2D tryptic peptide maps obtained from EC and embryo cells showed that this protein is similar, if not identical to SV40 induced p53 and is conserved evolutionarily. The p53 was also isolated and partially characterize from various tumorigenic mouse cells such as L cells, neuroblastoma cells, 3T12 cells. The p53 was also present in human placental cells. The turnover of this protein was found to be rapid in tumorigenic and in non-tumorigenic cells whereas in most SV40 transformed cells the protein was found to be stable due to its interaction and complex formation with the large T antigen of SV40. However, it was found that certain SV40 transformed mouse cells possess stable p53 not in complex with the T antigen. Also in SV40 transformed term and first trimester human placental cells the p53 is not in complex with the T antigen.